The separation columns used in the area of chromatography, for example high performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC) and capillary electro-chromatography (CEC), in many cases include very valuable packing material which should be protected against contaminants which can be absorbed in the packing material. These types of contaminants impair the separation accuracy of the separation column. However, the contaminants are very frequently contained in the fluid to be analyzed and cannot be removed in many cases by pure filtration. The use of a pre-column can provide a remedy in this connection. A pre-column is arranged upstream of the actual separation column and can include the same packing material as the separation column, however in many cases in smaller volumes. As a result, on the one hand the fluid to be analyzed is filtered and on the other hand it can also be concentrated.
Separation columns with a pre-column are described, for example, in DE 694 17 240 T2 and in DE 697 15 437 T2. The fluid to be analyzed is guided via capillary tubes to the columns, the capillary tubes being connected to the respective columns by way of connecting devices, frequently designated as fittings. In DE 694 17 240 T2, the pre-column is arranged in an adapter housing which is screwed into a socket unit of the connecting device so that no further capillary tube is required, as is the case, for example, in DE 31 15 873 A1.
Generally to be observed is a tendency toward packing materials with ever decreasing particle sizes, as a result of which the available surface-volume ratio is enlarged, which, in turn, increases the separation accuracy. However, as a result pressure loss in the separation column and the pre-column increases, as a result of which ever increasing pressures are required so that the fluid to be analyzed is able to run through the separation column and the pre-column. In this case, the sealing of the capillary tube in the region of the fittings poses a particular problem. In many cases the fittings comprise a so-called cutting ring screw connection which consists of a cap nut, a clamping cone and a cutting ring. The capillary tubes are pushed into a pilot bore of the fittings. As a result of tightening the cap nut, which tapers conically inward, the cutting ring is compressed, as a result of which its wedge-shaped ring inside cuts into the wall of the capillary tube and produces a tight positive locking connection. Due to its design, in this connection the outside diameter of the capillary tubes has to be somewhat smaller than the diameter of the pilot bore such that a small gap is formed here. When viewed from the end of the capillary tube which is pushed into the pilot bore, the clamping core is situated somewhat set back such that a certain dead volume is produced around the end of the capillary tube, which results in the fluid to be analyzed being carried over which has a negative effect on the separation accuracy of the columns. In addition, the fluid to be analyzed can still be contaminated by the fluid analyzed beforehand.
The problem of the dead space of the cutting ring screw connection is made even worse as a result of all the components having to be matched to one another. No cutting ring screw connections can be used for fittings with different pilot bore lengths without increasing the dead space further.